Numerous and varied configurations for using the energy of the sun to heat a fluid have been practiced. The greatly increased cost of other forms of energy has recently given new impetus to devising more inexpensive and efficient ways to harness and use solar energy. In the field of heating water in a tank by means of solar energy, a substantial portion of the implemented designs employ previously developed technology and materials. Solar water heating systems have been provided which use a solar heated anti-freeze fluid which is carried to a heat exchanger in a closed loop system. The closed loop system is generally characterized as a system in which the transport or conveying pipes constantly contain fluid. This type of solar heating system greatly minimizes the risk of the fluid freezing in the transport pipes when the flow of the fluid is halted. On the other hand, the cost of anti-freeze and periodic inspection and maintenance requirements associated therewith greatly increase the expense of the heating system. In addition, the toxicity of the anti-freeze in proximity with the water to be heated, albeit separated by presumably impervious barriers, provides at least some risk of an unwanted mixing of toxic anti-freeze and potable water. To minimize this risk, recent building codes require a double thickness heat exchanger which considerably reduces the heat exchange efficiency. Other solar systems circumvent the need for an anti-freeze fluid by draining the fluid from the outdoor located solar collector and transport lines when the system is not operating and the fluid is not moving through the transport lines. This is accomplished by means of gravity and by placement of portions of the transport lines above the level of the solar heated working fluid contained in a storage tank. In this draining embodiment, however, an air vent in the conveying lines is usually provided. As a consequence, air is constantly mixed with the fluid. The oxygen in the air can cause the inner wall of the storage tank and other bare steel components to corrode or rust. Additionally, the pumping mechanism which drives the fluid through the conveying lines can also corrode. Stainless steel materials can be used to obviate the problem of rust. But this material substantially increases the cost of the water heating system. In another alternative design, a solar water heating system includes a pair of fluid containing tanks. A rigid inner tank is surrounded by a rigid outer tank with a space therebetween for containing solar heated water. The wall of the inner tank acts as a heat exchanger for receiving the solar energy collected by the solar heated water. The inner tank contains the water to be heated which is then dischargable when desired for use. Both the inner tank and outer tank are fixed in shape and must be constructed of sufficiently strong and thick materials. The inner tank is subject to pressure from the incoming pressurized water flowing into the inner tank. The outer tank must be capable of withstanding the increasing pressure provided by the air space located above the level of the water in the outer tank which expands when heated. Again, the use of two relatively thick, rigid tanks increases the cost of the system. In addition, the possibility of the inner tank leaking and thereby adding water to the outer tank presents the risk of additional pressure within the outer tank which may cause a rupture of the outer tank.
The solar water heater of the present invention seeks to minimize the cost thereof, while maintaining the requisite efficiency inherent in an inner-outer tank configuration, and yet overcoming the aforementioned cost, pressure and corrosion problems. Basically, the present invention includes a variable volume outer tank which continuously minimizes the pressure resulting from the heated solar water. The outer tank also has a transport or conveying line connected thereto to permit movement of the air from the air space above the level of the solar heated water to the transport line in the solar collector for replacing the fluid therein. Yet, no air outside of the heating system is introduced into the conveying line. As a result, corrosion of the system components is also minimized.